This invention relates, in general, to photodetectors, and more particularly, to p-i-n photodiodes having increased absorption efficiency of incident light.
P-i-n photodiodes have been designed in either a vertical or a horizontal configuration. Vertically configured p-i-n photodiodes or photodetectors have been successfully manufactured for a number of years. However horizontally configured p-i-n photodiodes have only been demonstrated in a research environment.
Utilization of vertical high band-gap p-i-n photodiodes as components in discrete form or part of a monolithic process is important for optical transmission systems operating in a range of 10 giga bits per second. Bandwidths of these devices at long wavelengths, 1.3 to 1.55 micrometers, have achieved a 3 decibel bandwidth up to a frequency of 67 gigahertz using a mesa structure for vertical illumination with an indium gallium arsenide absorbing region lattice matched and epitaxially grown on an indium phosphide substrate. Increasing performance demands have been predicted with frequency values as high as 200 gigahertz. However, this performance level has not yet been achieved.
As increasing performance demands and smaller size restrictions are made on vertical p-i-n photodiodes several problems occur. Bandwidth, quantum efficiency, and fiber optic alignment are three examples of problems caused by increasing performance demands. Bandwidth and quantum efficiency are both effected by device design, structure, and conventional manufacturing techniques. Usually, as a broader bandwidth is required, quantum efficiency decreases in vertically configured p-i-n photodiodes. Also, as photodetector devices get smaller in size to attain higher performance it is more difficult to align fiber optics to the photodetector.
P-i-n photodiodes consist of a simple circular mesa structure that has been successfully used in long wavelength optical communication systems in a discrete form, as well as, in the shorter wavelength region (0.5-1.2 microns). The p-i-n photodiode has been able to achieve high reliability, hybrid integration with low voltage amplifiers, low noise levels, and low leakage currents.
Horizontally illuminated p-i-n photodiodes have shown some promise for high performance photodetectors but, have severe fiber optic alignment problems. These photodiodes are illuminated from a side facet with extremely high precision fiber optics. The alignment of fiber optics for horizontal photodiodes is very difficult to achieve and expensive to do. The high precision required to align fiber optics to a horizontal photodiode prohibits using this type of photodetector, and this has restricted this type of device to the research laboratory.
It is evident that conventional methods of manufacturing p-i-n photodiodes have severe limitations as performance levels increase. Therefore, a method for improving performance of vertical p-i-n photodiodes by increasing bandwidth, quantum efficiency, and fiber optic alignment tolerances would be highly desirable.